Engine fuel may be pumped out of a fuel tank by a lift pump. The lift pump propels fuel towards a fuel rail before being injected by fuel injectors. A check valve may be included between the lift pump and the fuel rail to maintain fuel rail pressure and prevent fuel in the fuel rail from flowing back towards the lift pump. Operation of the lift pump is typically feedback controlled by an engine controller based on outputs from a pressure sensor coupled in the fuel rail. The controller attempts to maintain the pressure in the fuel rail to a desired pressure by adjusting an amount of power supplied to the lift pump based on a difference, or error, between the desired fuel pressure and a measured fuel pressure obtained from the pressure sensor.
However, the inventors herein have recognized potential issues with such systems. As one example, when the fuel injectors are turned off, such as during deceleration fuel shut-off (DFSO), power to the lift pump may be reduced. Turning off the fuel injectors may cause fuel rail pressure to increase while the lift pump is on and spinning. Thus, power to the lift pump, and therefore lift pump speed may be reduced in an attempt to reduce fuel rail pressure. However, since fuel is prevented from flowing backwards through the check valve, reducing power to the fuel pump may have no effect on the fuel pressure of fuel included between the check valve and the fuel rail. Further, when fuel injection is commanded back on, it may take time for the fuel pump to spin up. Due to the delay of the fuel pump spin-up time, and/or integrator wind-up of the controller, transient fuel pressure drops may occur when exiting DFSO, leading to fuel metering errors that may degrade engine thermal efficiency and increase regulated emissions.
Further, in examples where the fuel rail pressure is variable, closed loop control of the lift pump may command for a decrease in lift pump voltage when fuel injection is insufficient to lower the fuel rail pressure at a desired rate. However, since decreasing lift pump voltage may have little to no effect on fuel rail pressure, such closed loop control of the lift pump may result in wind-up of the integral term and transient pressure undershoots.
As one example, the issues described above may be addressed by a method comprising closed loop operating a lift pump of a fuel system based on a difference between a desired fuel rail pressure and an estimated fuel rail pressure, and open loop operating the lift pump to the desired fuel rail pressure in response to a fuel flow rate in a direction of a fuel rail through a check valve positioned between the lift pump and the fuel rail decreasing to a threshold.
During the closed loop operating the lift pump, an amount of power supplied to the lift pump may be adjusted based on outputs from a pressure sensor coupled in the fuel rail. Specifically, the closed loop operating the lift pump may comprise adjusting an amount of power supplied to the lift pump based on one or more of a proportional term, integral term, and derivative term. Updating and computing the proportional term and integral term may comprise calculating an error based on a current difference between the desired fuel rail pressure and a most recently estimated fuel rail pressure obtained from the pressure sensor. However, open loop operating the lift pump may comprise adjusting the amount of power supplied to the lift pump based only on the desired fuel rail pressure and not based on outputs from the pressure sensor. Specifically, open loop operating the lift pump may comprise freezing the integral term and clipping the proportional term to non-negative values.
In another example, a method for an engine may comprise adjusting an amount of power supplied to a lift pump of a fuel system based on a difference between a desired fuel rail pressure and an estimated fuel rail pressure of a fuel rail, and regulating the amount of power supplied to the lift pump based on a desired lift pump outlet pressure in response to a fuel flow rate in a direction of the fuel rail through a check valve positioned between the lift pump and the fuel rail decreasing to a threshold.
In yet another example, an engine system may comprise a lift pump, a fuel rail including one or more fuel injectors for injecting liquid fuel, a check valve positioned between the lift pump and the fuel rail, a pressure sensor coupled to the fuel rail, and a controller including non-transitory memory with instruction for: switching from closed loop control of the lift pump to open loop control in response to a fuel flow rate through the check valve decreasing to a threshold, and resuming closed loop control of the lift pump in response to the fuel flow rate through the check valve increasing above the threshold.
In this way, transient pressure drops in the fuel rail may be reduced. Specifically, by open loop operating the lift pump during DFSO, lift pump speed may be maintained at a higher level than it would be under closed loop control during DFSO. As such, lift pump spin-up time when exiting DFSO may be reduced, and pressure drops in the fuel rail may be reduced. Thus, fluctuations in fuel rail pressure may be reduced and fuel rail pressure consistency may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.